The present invention relates to body implantable devices and more particularly to prostheses incorporating the characteristics of stents and grafts and intended for long term intraluminal fixation.
A variety of patient treatment and diagnostic procedures involve devices intraluminally implanted into the body of a patient. Among these devices are stents as disclosed in U.S. Pat. No. 4,655,771 (Wallsten). The devices in Wallsten are tubular, braided structures formed of helically wound thread elements. The stents are deployed using a delivery catheter such as discussed in U.S. Pat. No. 5,026,377 (Burton et al.). With the stent positioned at the intended treatment site, an outer tube of the delivery catheter is withdrawn allowing the prosthesis to radially expand into a substantially conforming surface contact with a blood vessel wall or other tissue.
Thread elements or strands formed of metal are generally favored for applications requiring flexibility and effective resistance to radial compression after implantation. Metal strands can be thermally formed by a moderately high temperature age-hardening process while wound about a mandrel in the desired helical configuration. The strands, due to their high modulus of elasticity, cooperate to provide the needed strength. Strand flexibility also permits a radial compression and axial elongation of the stent that facilitates intraluminal delivery of the stent to the intended treatment site. Because the self-expanding device generally remains at least slightly radially compressed after fixation, its elastic restoring force can provide acute fixation.
The favorable combination of strength and flexibility is due largely to the properties of the strands after they have been age-hardened, or otherwise thermally treated in the case of polymeric strands. The braiding angle of the helical strands and the axial spacing between adjacent strands also contribute to strength and flexibility. Age hardening processes are described in U.S. Pat. No. 5,628,787 (Mayer) and U.S. Pat. No. 5,645,559 (Hachtman et al.).
A well known alternative stent construction features plastically deformable metallic strands in lieu of resilient strands. Plastically deformable strands can be arranged in the same helical configuration. A plastically deformable stent requires no gripping members or other feature on the catheter to maintain the stent in a reduced-radius state during delivery. However, radial expansion of the stent at the treatment site requires a dilatation balloon or other expansion means.
Regardless of whether stents are self-expanding or plastically deformable, they characteristically have an open mesh construction, or otherwise are formed with multiple openings to facilitate the necessary radial enlargements and reductions and to allow tissue ingrowth of the metallic structure. Also, such stents characteristically longitudinally expand as they radially contract, and conversely radially expand as they longitudinally contract.
Devices featuring more tightly woven strands are known. For example, U.S. Pat. No. 4,681,110 (Wiktor), discloses a flexible tubular liner insertable into the aorta to treat an aneurysm. The liner is a tight weave of flexible plastic strands, designed to elastically expand against the aneurysm to direct blood flow past the aneurysm. The tight weave is intended to minimize leakage, so that the liner can effectively shunt blood through to eliminate the aneurysmal sack from the blood path.
The Wiktor structure and others like it notwithstanding, those of skill in the art continue to encounter difficulty in providing a device that simultaneously accommodates the competing needs of low permeability, strength and flexibility for radial compression and expansion. One known approach to this problem is a combination stent graft, in which a compliant but substantially fixed-radius and tightly woven graft is sutured or otherwise coupled to a radially expandable stent. Upon release, the stent is intended to radially expand to the graft diameter. This requires a careful matching of the graft diameter with the lumen diameter at the treatment site. Otherwise, either an oversized graft is compressed between the stent and body tissue with undesirable folds or gathering of the graft material, or an undersized graft prevents the stent from radially expanding an amount sufficient to anchor the device.
Another difficulty arises from the fact that the stent layer and graft layer, even when both undergo combined radial contraction and axial elongation, behave according to different relationships governing the amount of radial reduction for a given axial increase. When the latticework elongates a greater amount for a given radial reduction, elongation of the composite structure tends to tear the bond joining the graft material to the stent. Conversely, if the graft layer undergoes the greater axial expansion, an unwanted increase in bending stiffness causes localized reductions in diameter when the stent graft is bent around tight radii. Consequently negotiation through tortuous vascular passageways becomes more difficult, and in some instances impossible.
The commercially available yarns used in textile vascular grafts are twisted for improved handling during weaving or knitting operations. The amount of twisting will depend upon certain factors including the process of yarn manufacture (e.g., continuous filament yarn or staple yarn) and desired denier. For continuous filament yarn processes, surface twisting angles will generally be between about 15-45 degrees. The multiple filaments typically form a substantially circular yarn cross-section. This limits the effectiveness of the stent graft, and increases the difficulty of matching the elongation behavior of the fabric graft, to that of the stent.
More particularly, the twisted multifilaments are tightly packed, yielding packing factors (or packing ratios) in the range of 0.7-0.9. Because of the tightly packed yarns, the tubular fabric graft has a tendency to kink when bent. The tightly packed filaments leave an insufficient void throughout the yarn cross-section for tissue ingrowth, reducing the effectiveness of long-term fixation. Further, the tightly packed yarn cross-section does not adjust itself in shape to accommodate axial elongation, thus limiting the radial contraction/axial elongation capability of the graft. The circular yarn cross-section further limits the elongation capability, because of its particular resistance to adjustments in shape.
Other disadvantages arise from the circular yarn cross-section. The yarn diameter determines the minimum thickness of the graft fabric. Yarn coverage typically is below 80 percent without additional compacting, and a fabric porosity usually is above 70 percent, again without additional compacting.
Several prostheses constructions had been suggested for composite braided structures that combine different types of strands, e.g. multifilament yarns, monofilaments, fusible materials and collagens. Examples are found in International Patent Publications No. WO 91/10766, No. WO 92/16166, No. WO 94/06372, and No. WO 94/06373. Further, a highly favorable combination of strength, resilience, range of treatable lumen diameters and low permeability has been achieved by two-dimensionally woven and three-dimensionally woven composite devices featuring textile strands interbraided with either selectively cold-worked or independently thermally set structural strands, as disclosed in U.S. patent applications Ser. No. 08/640,062, now U.S. Pat. No. 5,758,562, and Serial No. 08/640,091, now U.S. Pat. No. 5,718,159, both filed Apr. 30, 1996 and assigned to the assignee of this application. Although such devices are well suited for a wide range of procedures, there are costs and complexities inherent in interweaving different types of strands. Certain desirable modifications, e.g. providing selected areas of the device with only structural strands, are difficult.
All references cited herein, including the foregoing, are incorporated herein in their entireties for all purposes.
Therefore, it is an object of the present invention to provide a prosthesis structure that affords the advantages of stents and grafts, yet does not require an interbraiding of the structural strands characteristic of stents and the textile strands characteristic of grafts.
Another object is to provide a process for manufacturing a combination stent graft in which a structural layer and a low-permeability fabric layer undergo radial and axial enlargements and reductions, yet remain integrally bonded to one another.
It is a further object of the invention to provide a stent graft construction that facilitates selective alternate axial positioning of open-mesh areas and covered areas for shunting blood flow, to customize stent grafts for particular uses. Yet another object is to provide, in a prosthesis featuring two or more layers formed of braided strands and of different materials, an effective bond to ensure that the layers remain integrally connected through radial expansions and contractions. Another object is to provide a fabric graft that exhibits low permeability due to a high yarn coverage and low fabric porosity, in combination with a yarn cross-sectional porosity sufficient to enable tissue ingrowth.
A further object is to provide a stent graft with a fabric graft that is thinner and more closely matches the elongation behavior of the stent, yet affords acceptably low permeability.